This feature has already been implemented, or is already achievable with existing methods.

(Dream Daemon): When hosting a game have an option to host for subscribers only

(BYOND Pager): When a subscriber's only world is being host & you are a subscriber
you are told that there is a live game while fans (not subscribers) of the game are not told

Can be good for private testing
Example:
I host UPC Codes Attack for testing but I only want 10 players
I make the 10 subscribers & host a subscriber's only world & only the 10 players I want will see I'm hosting a game

host var (world)
See also:
game_state var (world)
name var (world)
status var (world)
visibility var (world)
Default value:
null
If the information is made available by the pager, this will provide the key of the world's host. If the host is not known, this value will be either null or an empty string.